In the manufacture of many commercial products, a liquid composition is applied as a coating to a receptor substrate. In many applications, and especially in photographic films and papers, the requirements for areal uniformity of coated thickness are highly demanding. Limitations on thickness variation of 1% or even less can be required. Density variations will form in the coating and when a uniform exposure is given to different density levels, streaks and non-uniform images are obtained.
Known coating apparatus typically includes a backing roller around which a web to be coated is wrapped and conveyed at a predetermined conveyance speed. A liquid composition is continuously delivered to and reshaped by an applicator, generally known as a hopper, from a jet flow at the applicator inlet into a broad ribbon of substantially uniform thickness at the applicator outlet from which it is dispensed onto the moving web. Typically, such an applicator is positioned either immediately adjacent to the moving web at a distance of typically less than 1 mm, a transverse, dynamic bead of composition being formed therebetween (bead coating), or above the web at a distance of typically several cm, the composition being allowed to fall as a curtain under gravity into continuous contact with the moving web (curtain coating). A liquid composition may be a single layer or a composite layer consisting of a plurality of coating compositions.
In all coating systems, there is an upper speed limit for coating at which the boundary layer of air carried on the surface of the web is no longer squeezed out by the advancing composition at the coating point but rather becomes entrained under the composition, disrupting the uniform application thereof to the web and resulting in unacceptable coating uniformity.
It is well known that electrostatic charging of a coating apparatus by electrifying the surface of the coating roller can be useful in increasing the upper limit of coating speed. See, for example, U.S. Pat. Nos. 3,335,026 issued Aug. 8, 1967; 4,837,045 issued Jun. 6, 1989; and U.S. Pat. No. 4,864,460 issued Sep. 5, 1989. However, the web can have random charge patterns created prior to the location of the ionizers, due to the unwinding and conveyance process as well as corona discharge treatment of the web. As charge nonuniformities on the web when entering upon the charged coating roller are not neutralized, or smoothed, by the charged roller but simply added to the electrostatic field imparted by the roller, the areal charge nonuniformity can result in a corresponding coating nonuniformity.
Methods and apparatus have been proposed to enhance coatability by removal of charge nonuniformities from both surfaces of a web by neutralizing charges on the web ahead of the coating roller. See U.S. Pat. Nos. 3,470,417 issued Sep. 30, 1969; U.S. Pat. No. 3,531,314 issued Sep. 29, 1970; U.S. Pat. No. 3,730,753 issued May 1, 1973; and 5,432,454 issued July 11, 1995. Such proposals avoid the problem of web charge nonuniformities created by processes such as corona discharge treatment but do not deal with the problem of providing an electrostatic assist to enhance web coatability and increase coating speed.
It is also well known that electrostatic charging of a web can be useful in increasing the upper limit of coating speed. For example, a dielectric web carrying a bound polar charge between opposite surfaces thereof can exhibit increased "wettability" and a consequent increase in acceptable coating speed when conveyed around a grounded coating roller. Means for applying such a charge to a web ahead of the coating point are disclosed, for example, in European Patent No EP 390774 issued Jul. 15, 1992; U.S. Pat. No. 4,835,004 issued May 30, 1989, U.S. Pat. No. 5,122,386 issued Jun. 16, 1992, U.S. Pat. No. 5,295,039 issued Mar. 15, 1994; and European Patent Application No. 0 530 752 A1 published Mar. 3, 1993.
Serious problems can arise in using electrostatic assist for coating in processes wherein the web is charged ahead of the coating point. For example, it can be difficult to apply the charge uniformly over the web. Ionizers must be rigorously maintained, and charging webs at high speeds can require prohibitively large and expensive installations. Apparatus and methods have been proposed for correcting the charge nonuniformity that can occur during the charge application process. See, for example, U.S. Pat. No. 4,835,004 and European Patent No. 0 530 752 A1 which propose to control charge uniformity by imposing strict environmental controls around the web. Such controls can be expensive to install and operate and also may be only marginally effective as heat and humidity are used to aid in the electrostatic assist by smoothing the charges and not removing them. This environmental control should not be required. Environmental control, such as heating the web, relies on changes in physical and electrical properties as the web. These changes can limit the choice of webs and/or sorting on these webs.
Further, even when charge has been applied uniformly, the uniformity can be compromised by any of various well known contacts or exposures between the charge application point and the coating point. It has been observed that a significant loss in charge from a charged web surface can occur upon contact with conveyance rollers that typically are conductive and electrically grounded. These rollers may have a surface pattern such as a series of circumferential grooves to provide traction. The charge loss experienced by a charged web surface when conveyed over these rollers occurs in a manner corresponding to the surface pattern. Areal variations in charge on the web when it reaches the coating point typically result in variations in layer thickness and consequent density nonuniformity also corresponding to the surface pattern. None of the prior art discusses this charge loss issue between the charge application point and the coating point.
Further, charge remaining on the web after coating can be a shock hazard to operators and can be a marking or fogging hazard to light-sensitized product later in coating and in finishing.
Thus there is a need for a method for coating a liquid composition to a moving web at high speed whereby the web is rendered substantially discharged ahead of the coating point, and whereby the coating bead or curtain is subjected to a highly uniform electrostatic field widthwise of the web at the point of coating.